This invention pertains to liquid compositions of urea, ammonia, and water, and most particularly to liquid fertilizer compositions of urea, ammonia, and water containing relatively high concentrations of total nitrogen content.
Aqueous solutions of ammonia are well-known liquid fertilizers, as are aqueous solutions of urea. Although it would be desirable to prepare such solutions in high concentrations, the practical limit of total nitrogen content for solutions of either ammonia or urea is about 20 percent by weight, but for different reasons. In the case of aqueous ammonia, exceeding the 20 weight percent limit runs into vapor pressure problems, i.e., the vapor pressure of the solution is so high as to cause boiling or vaporization at a temperature within or below an expectable range of ambient temperatures, so that exposure to the atmosphere will cause vaporization and consequent loss of ammonia. Moreover, even if stored in a closed vessel under pressure, much ammonia will still be vaporized and lost when the solution is reduced in pressure upon application to the soil. On the other hand, urea solutions of above 20 weight percent total nitrogen content have solubility problems under cold weather or relatively cold weather conditions. Specifically, highly concentrated urea solutions begin to crystallize or "salt out" at temperatures likely to be encountered in typical agricultural situations. As a result, use of concentrated urea solutions risks unacceptable changes in liquid nutrient content, clogging of delivery systems, changes in viscosity, and unreliability in measuring precise quantities of dissolved fertilizer.
It would, of course, be highly advantageous in agricultural situations if a liquid fertilizer composition were available of relatively high total nitrogen content, i.e., above about 30 weight percent, particularly since the higher the nutrient content, the lower the water content, and thus the greater the amount of nitrogen-containing nutrients which can be held in a given volume and the lower the energy cost in transporting and spreading a given weight of such nutrients. But for the reasons pointed out hereinbefore, solutions containing either urea or ammonia are not agronomically successful when prepared in relatively high total nitrogen contents, and efforts toward the production of fertilizers containing both ammonia and urea in relatively high total nitrogen content have been discouraged--and this despite the fact that combined ammonia-urea solutions would have the advantage of providing both a "contact" fertilizer (i.e., ammonia) that remains fixed at the point of application in the soil and a "mobile" fertilizer (i.e., urea) that travels with the water carrier for about two days.
The reason combined urea-ammonia aqueous solutions have not been prepared in relatively high nitrogen contents may be explained by reference to FIG. 1 of the accompanying drawing, which depicts a ternary phase diagram of the water-urea-ammonia system presented by Worthington, Datin, and Schutz in "Physical Properties of Ammonia Solutions," Industrial and Engineering Chemistry, p. 911 (April, 1952). As shown on this phase diagram, as the various isothermal crystallization curves move from right to left and away from the urea-water ordinate in the direction of increasing total nitrogen content, the curves first reach minimum values, then rise toward inflection points, and finally fall off towards the urea-ammonia ordinate. For agricultural purposes, the crystallization curves set forth in FIG. 1 present a dilemma. In the regions wherein crystallization and "salting out" problems are minimized, i.e., generally in the left-hand portion of the graph, and especially in the areas directly under the inflections, it is known that vapor pressures are unacceptably high, so that solutions of compositions defined by concentrations falling within this region would require heavy gauge handling materials and would boil upon exposure to the atmosphere, for example, when held in a tank open to the atmosphere or when applied to the soil. On the other hand, where the vapor pressures are known to be more favorable, i.e., generally in the right-hand portion of the diagram, Worthington et al. predict that the worst solubility conditions exist, and that "salting out" problems will be encountered, with the problems being most acute in the region where the curves reach minimum values.
In addition, the curves in FIG. 1 predict a further disadvantage if one is interested in liquid urea-ammonia-water fertilizers of high urea content. Specifically, although the isothermal crystallization curves run from right to left (which is the direction of increasing total nitrogen content), the minimum values that the curves first reach reveal, if solubility and vapor pressure problems are to be avoided, that the total nitrogen content can only be increased by decreasing the urea content and increasing the ammonia content--a result which would clearly be detrimental if a urea-ammonia-water solution of high "mobile" nitrogen is desired
For the foregoing reasons, therefore, liquid urea-ammonia-water fertilizer compositions of relatively high total nitrogen content have been avoided, and aqueous solutions containing either ammonia or urea but not both remain the predominant form of nitrogen-containing liquid fertilizers. These fertilizers, although useful, have not only the disadvantage mentioned hereinbefore--i.e., a practical maximum total nitrogen content of 20 weight percent--but the further disadvantage of requiring substantial heating or cooling facilities for their preparation. More specifically, since the dissolution of ammonia into aqueous media involves exothermic reactions, cooling equipment such as condensers and the like are required for the preparation of aqueous ammonia fertilizers. On the other hand, urea-water reactions are highly endothermic, and consequently, the preparation of aqueous liquid urea fertilizers requires heating means such as boilers and the like.
In view of the drawbacks associated with aqueous liquid fertilizers of either urea or ammonia, it can be seen that there is a need for an aqueous fertilizer of higher total nitrogen content than is commercially possible with urea or ammonia, and a further need for producing such a fertilizer with a minimum energy expenditure for heating and/or cooling. Further still, a need exists for providing a fertilizer whose crystallization and boiling points are such that the differential therebetween is attractive agronomically, allowing for storage, transportation, and application to the soil under atmospheric pressure and within a relatively wide range of ambient temperatures.
Accordingly, it is an object of the present invention to provide liquid urea-ammonia-water fertilizers having a relatively high total nitrogen content, i.e., above about 30 percent by weight, and further having an unexpectedly low crystallization temperature.
It is an especial object of the invention to provide liquid fertilizer compositions comprising urea, ammonia, and water which contain a relatively high total nitrogen content and have an unexpectedly high differential between their crystallization and boiling point temperatures.
It is still another object to provide such compositions having the yet further advantage of higher urea content than heretofore believed possible.
In addition, another object of the invention is to provide a method for storing or transporting such liquid fertilizer compositions in closed vessels under essentially atmospheric pressure, and also a method for applying such fertilizers to the soil without substantial loss due to crystallization or vaporization.
And yet another object of the invention is to provide a method for preparing liquid urea-ammonia-water compositions while minimizing energy expended for heating and/or cooling, with still another object being to provide a fertilizer of superior properties for a given distribution of absolute total nitrogen to the soil.
These and other objects of the invention will become apparent to those skilled in the relevant art in view of the following description of the invention.